1. Field of the Invention
The present invention relates to vibration monitoring equipment and more specifically to proximity probe sensors used to monitor vibrations in motors and shafts and to identify excessive vibrations therein.
2. Prior Art
Rotating machinery such as motors, generators, and turbines find widespread application in areas such as manufacturing, power generation, materials processing, as well as many others. Over time, such machinery is subject to wear and potential failure. Given the high operating RPM and high power dissipation of many industrial applications of such machinery, failure during operation may have severe consequences in terms of damage to the failed equipment itself as well as neighboring equipment and areas of the installation. In addition, preventive machinery shut downs for maintenance and repair can be very costly in terms of facility downtime and direct expense in labor and replacement parts.
Accordingly, it has become important in this area to provide monitoring equipment associated with such rotating machinery to provide indications of its condition. Preferably, such condition monitoring equipment can indicate when a piece of machinery is excessively worn or is otherwise operating improperly. In these instances, the affected machinery can be shut down and repaired prior to a catastrophic failure. Furthermore, such monitoring equipment can indicate when machinery is operating within a defined normal operating range, thereby eliminating unnecessary shutdowns for preventive maintenance. It can be appreciated that condition monitoring equipment employed for these purposes should quickly detect and either flag or shut down equipment subject to imminent failure, but should not unnecessarily flag or shut down equipment that is functioning properly.
Two parameters monitored by such equipment are the amplitude and frequency of the machine vibration. A typical rotating machine will have vibration at many frequencies. By monitoring both the amplitude and frequency of the machine vibration one can determine the condition of the machine as well as provide protection from catastrophic failure.
One type of condition monitoring device is a two wire proximity transmitter system that converts a non-linear eddy current signal from an eddy current probe into a linearized output suitable for conversion into a 4 to 20 mA process signal. This signal is used to indicate whether a portion of a rotating machine such as a motor or compressor is in need of repair or maintenance. It can also be used to provide protection to the machine by indicating excessive vibration levels, thus allowing an external control system to shut the machine down.
Eddy current proximity probe systems, and particularly, eddy current probes are well known for their ability to detect the position or condition of varying types of conductive materials. These probes are useful in a variety of related applications including position measurement (such as axial and radial runout or displacement of a rotating assembly) and defect or flaw detection in metallic objects. For example, eddy probe systems are commonly used to detect the lateral position of a rotating shaft in relation to its journal bearing by mounting one or more probes within the bearing in close proximity to the shaft.
Eddy current probes comprise an inductor, or coil, situated at the probe tip driven with a radio frequency (RF) signal which in turn creates a varying magnetic field in any adjacent conductive target material. This magnetic field produces eddy currents in the material that induce a counter-electromotive force (emf) in the eddy probe inductor, thereby altering the effective impedance of the inductor. The impedance of the probe therefore provides an indication of the distance between the target and the probe. Varying the distance of the conductive target element, i.e., the motor shaft, from the coil varies the impedance of the detector coil and thereby varies the output frequency and voltage of the oscillator.
Typically, the impedance output of the probe is not used directly to monitor operating conditions. This is because the impedance of the probe coil is not linearly related to the distance between the coil and the target surface. This can be inconvenient for automated target monitoring because depending on the region of the impedance vs. distance curve the system is operating at, small changes in impedance may indicate much larger changes in distance, or alternatively, large impedance changes may indicate small distance changes.
Thus, rather than compute target position directly from impedance measurements, it is common in the condition monitoring industry to transform the non-linear impedance measurements into a second analog signal, such as 0 to −24 Vdc or 4 to 20 mA, which is linear with target distance. More specifically a two wire proximity transmitter converts the non linear eddy current signal from an eddy current probe into a linearized output suitable for conversion into a 4 to 20 mA process signal. This signal is used to indicate whether a portion of a rotating machine such as a motor or compressor is in need of repair or maintenance. It can also be used to provide protection to the machine by indicating excessive vibration levels, thus allowing an external control system to shut the machine down. The transformation of the eddy current probe output into a linear analog signal suitable for host monitoring equipment has been typically accomplished using a linearization table that is programmed into the transmitter. Such linearization tables are standardized across a range of probe systems, and do not take into account the specific characteristics of a given probe and a given cable. In other words, the same table is typically downloaded into all transmitters without regard to the differences that may exist from system to system.
In some prior art probe systems, a phase locked loop (PLL) oscillator circuit may be used to maintain resonance of an eddy current probe with varying equipment configurations, conductive target materials, and target displacements. A phase locked loop circuit is a circuit which synchronizes an adjustable oscillator with another reference oscillator by the comparison of phase between the two signals. Essentially, the PLL technique is used to stabilize the generated signal used to drive the eddy current probe. Those skilled in the art will appreciate that a PLL is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal. In addition to stabilizing a particular communications channel (keeping it set to a particular frequency), a PLL can be used to generate a signal, modulate or demodulate a signal, reconstitute a signal with less noise, or multiply or divide a frequency.
Two wire eddy current probe position monitoring devices typically have the disadvantage that the power available is limited by the minimum supply voltage and the minimum current consumption. This amount is in the 35 to 40 mW range. One drawback to analog eddy current proximity probes systems of the prior art is that such systems require a complex circuit which is difficult to manufacture and tune, and has an accuracy which is difficult to maintain. Furthermore, individual eddy current probe and cable configurations and target materials require individual tuning of one or both of the oscillator circuit biasing and the variable gain amplifier. More specifically, instrumentation personnel are faced with issues such as transmitter consistency when replacing a broken unit, dependable response over temperature and the need to sometimes maintain several versions of the transmitters for different machines. Further, prior art analog circuits require several component changes to handle different full scale ranges and probe types. These circuits are also prone to performance problems due to component variations. Component variations also required skilled technicians to troubleshoot circuits that had gone out of a proper operating range. This required much time and effort. Prior art circuits were also prone to more drift over temperature which required additional circuitry to compensate for these drifts. This generally limited the temperature range of the transmitter. The need to reduce the time necessary to build a particular transmitter and a way of improving the reliability and quality of the product was the impetus for the new design.
A drawback to PLL systems of the prior art is that these two wire devices typically consume a proportionally large amount of power. Typically, vibration monitoring systems are limited by the minimum supply voltage and the minimum current consumption. This amount is in the 35 to 40 mW range. Phase locked loop circuits can draw as much as 5 to 7 mW which is undesirable in cases where there is a maximum current consumption limitation.
It would therefore be highly desirable to provide a proximity probe/transmitter system that removes the need for a phase locked loop circuit and the electronic circuitry required to implement such a system. Such an improved transmitter would preferably eliminate the need for circuit modification due to equipment configuration changes and also preferably reduce the level of effort and increase the precision associated with system recalibration for different probe geometries and target materials.